A liquid crystal display device is widely utilized for a display of a personal computer, a television and so forth. The device utilizes optical anisotropy, dielectric anisotropy or the like of the liquid crystal compound. As an operating mode of the liquid crystal display device, various modes are known, such as a phase change (PC) mode, a twisted nematic (TN) mode, a super twisted nematic (STN) mode, a bistable twisted nematic (BTN) mode, an electrically controlled birefringence (ECB) mode, an optically compensated bend (OCB) mode, an in-plane switching (IPS) mode, a vertical alignment (VA) mode and a polymer sustained alignment (PSA) mode.
In such a liquid crystal display device, a liquid crystal composition having suitable physical properties is used. In order to further improve characteristics of the liquid crystal display device, the liquid crystal compound contained in the composition preferably has physical properties as represented in (1) to (8) below:
(1) high stability to heat, light and so forth;
(2) high clearing point;
(3) low minimum temperature of a liquid crystal phase;
(4) small viscosity (η);
(5) suitable optical anisotropy (Δn);
(6) large dielectric anisotropy (Δ∈);
(7) suitable elastic constant (K); and
(8) excellent solubility in other liquid crystal compounds.
An effect of the physical properties of the liquid crystal compound on the characteristics of the device is as described below. A compound having a high stability to heat, light and so forth as described in (1) increases a voltage holding ratio of the device. Thus, a service life of the device becomes long. A compound having a high clearing point as described in (2) extends a temperature range in which the device can be used. A compound having a low minimum temperature of a liquid crystal phase such as a nematic phase or a smectic phase as described in (3), particularly, a compound having a low minimum temperature of the nematic phase also extends the temperature range in which the device can be used. A compound having a small viscosity as described in (4) shortens a response time of the device.
A compound having a suitable optical anisotropy as described in (5) improves a contrast of the display device. According to a design of the display device, a compound having a large optical anisotropy or small optical anisotropy, more specifically, a compound having a suitable optical anisotropy is required. When shortening a response time by decreasing a cell gap of the display device, a compound having a large optical anisotropy is suitable. A compound having a large dielectric anisotropy as described in (6) decreases a threshold voltage of the display device. Thus, an electric power consumption of the display device becomes small. On the one hand, a compound having a small dielectric anisotropy, decreases a viscosity of the composition, and thus shortens a response time of the device.
With regard to (7), a compound having a large elastic constant shortens a response time of the display device. A compound having a small elastic constant decreases a threshold voltage of the display device. Accordingly, a suitable elastic constant is required according to characteristics to be desirably improved. A compound having an excellent solubility in other liquid crystal compounds as described in (8) is preferred. The reason is that physical properties of the composition are adjusted by mixing liquid crystal compounds having different physical properties.
Various kinds of liquid crystal compounds having a large dielectric anisotropy have been synthesized so far. The reason is that excellent physical properties that are not developed by a conventional compound are expected. The reason is that a suitable balance between two of physical properties required upon preparing the liquid crystal composition is expected for a new compound. Patent literature Nos. 1 to 7 describe a linear and cyclic compound having 2,2-difluorovinyloxy group.    Patent literature No. 8 describes a linear and cyclic compound (S-1) having a 1,3-dioxane ring.    Patent literature Nos. 9 to 12 describe compounds (S-2) to (S-5) having a CF2O bonding group and having a 2,2-difluorovinyloxy group.    Patent literature Nos. 13 to 14 describe compounds (S-6) to (S-7) having a bonding group other than a CF2O bonding group, and having a 2,2-difluorovinyloxy group.    Patent literature No. 15 describes compound (S-8).

In view of such a situation, a development is desired for a compound having excellent physical properties and a suitable balance with regard to the physical properties described in (1) to (8).